michael@0: var assert = require('assert');
michael@0: 
michael@0: // The Flow class
michael@0: // ==============
michael@0: 
michael@0: // Flow is a [Duplex stream][1] subclass which implements HTTP/2 flow control. It is designed to be
michael@0: // subclassed by [Connection](connection.html) and the `upstream` component of [Stream](stream.html).
michael@0: // [1]: http://nodejs.org/api/stream.html#stream_class_stream_duplex
michael@0: 
michael@0: var Duplex  = require('stream').Duplex;
michael@0: 
michael@0: exports.Flow = Flow;
michael@0: 
michael@0: // Public API
michael@0: // ----------
michael@0: 
michael@0: // * **Event: 'error' (type)**: signals an error
michael@0: //
michael@0: // * **setInitialWindow(size)**: the initial flow control window size can be changed *any time*
michael@0: //   ([as described in the standard][1]) using this method
michael@0: //
michael@0: // [1]: http://tools.ietf.org/html/draft-ietf-httpbis-http2-10#section-6.9.2
michael@0: 
michael@0: // API for child classes
michael@0: // ---------------------
michael@0: 
michael@0: // * **new Flow([flowControlId])**: creating a new flow that will listen for WINDOW_UPDATES frames
michael@0: //   with the given `flowControlId` (or every update frame if not given)
michael@0: //
michael@0: // * **_send()**: called when more frames should be pushed. The child class is expected to override
michael@0: //   this (instead of the `_read` method of the Duplex class).
michael@0: //
michael@0: // * **_receive(frame, readyCallback)**: called when there's an incoming frame. The child class is
michael@0: //   expected to override this (instead of the `_write` method of the Duplex class).
michael@0: //
michael@0: // * **push(frame): bool**: schedules `frame` for sending.
michael@0: //
michael@0: //   Returns `true` if it needs more frames in the output queue, `false` if the output queue is
michael@0: //   full, and `null` if did not push the frame into the output queue (instead, it pushed it into
michael@0: //   the flow control queue).
michael@0: //
michael@0: // * **read(limit): frame**: like the regular `read`, but the 'flow control size' (0 for non-DATA
michael@0: //   frames, length of the payload for DATA frames) of the returned frame will be under `limit`.
michael@0: //   Small exception: pass -1 as `limit` if the max. flow control size is 0. `read(0)` means the
michael@0: //   same thing as [in the original API](http://nodejs.org/api/stream.html#stream_stream_read_0).
michael@0: //
michael@0: // * **getLastQueuedFrame(): frame**: returns the last frame in output buffers
michael@0: //
michael@0: // * **_log**: the Flow class uses the `_log` object of the parent
michael@0: 
michael@0: // Constructor
michael@0: // -----------
michael@0: 
michael@0: // When a HTTP/2.0 connection is first established, new streams are created with an initial flow
michael@0: // control window size of 65535 bytes.
michael@0: var INITIAL_WINDOW_SIZE = 65535;
michael@0: 
michael@0: // `flowControlId` is needed if only specific WINDOW_UPDATEs should be watched.
michael@0: function Flow(flowControlId) {
michael@0:   Duplex.call(this, { objectMode: true });
michael@0: 
michael@0:   this._window = this._initialWindow = INITIAL_WINDOW_SIZE;
michael@0:   this._flowControlId = flowControlId;
michael@0:   this._queue = [];
michael@0:   this._ended = false;
michael@0:   this._received = 0;
michael@0: }
michael@0: Flow.prototype = Object.create(Duplex.prototype, { constructor: { value: Flow } });
michael@0: 
michael@0: // Incoming frames
michael@0: // ---------------
michael@0: 
michael@0: // `_receive` is called when there's an incoming frame.
michael@0: Flow.prototype._receive = function _receive(frame, callback) {
michael@0:   throw new Error('The _receive(frame, callback) method has to be overridden by the child class!');
michael@0: };
michael@0: 
michael@0: // `_receive` is called by `_write` which in turn is [called by Duplex][1] when someone `write()`s
michael@0: // to the flow. It emits the 'receiving' event and notifies the window size tracking code if the
michael@0: // incoming frame is a WINDOW_UPDATE.
michael@0: // [1]: http://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback_1
michael@0: Flow.prototype._write = function _write(frame, encoding, callback) {
michael@0:   if (frame.flags.END_STREAM || (frame.type === 'RST_STREAM')) {
michael@0:     this._ended = true;
michael@0:   }
michael@0: 
michael@0:   if ((frame.type === 'DATA') && (frame.data.length > 0)) {
michael@0:     this._receive(frame, function() {
michael@0:       this._received += frame.data.length;
michael@0:       if (!this._restoreWindowTimer) {
michael@0:         this._restoreWindowTimer = setImmediate(this._restoreWindow.bind(this));
michael@0:       }
michael@0:       callback();
michael@0:     }.bind(this));
michael@0:   }
michael@0: 
michael@0:   else {
michael@0:     this._receive(frame, callback);
michael@0:   }
michael@0: 
michael@0:   if ((frame.type === 'WINDOW_UPDATE') &&
michael@0:       ((this._flowControlId === undefined) || (frame.stream === this._flowControlId))) {
michael@0:     this._updateWindow(frame);
michael@0:   }
michael@0: };
michael@0: 
michael@0: // `_restoreWindow` basically acknowledges the DATA frames received since it's last call. It sends
michael@0: // a WINDOW_UPDATE that restores the flow control window of the remote end.
michael@0: Flow.prototype._restoreWindow = function _restoreWindow() {
michael@0:   delete this._restoreWindowTimer;
michael@0:   if (!this._ended && (this._received > 0)) {
michael@0:     this.push({
michael@0:       type: 'WINDOW_UPDATE',
michael@0:       flags: {},
michael@0:       stream: this._flowControlId,
michael@0:       window_size: this._received
michael@0:     });
michael@0:     this._received = 0;
michael@0:   }
michael@0: };
michael@0: 
michael@0: // Outgoing frames - sending procedure
michael@0: // -----------------------------------
michael@0: 
michael@0: //                                         flow
michael@0: //                +-------------------------------------------------+
michael@0: //                |                                                 |
michael@0: //                +--------+           +---------+                  |
michael@0: //        read()  | output |  _read()  | flow    |  _send()         |
michael@0: //     <----------|        |<----------| control |<-------------    |
michael@0: //                | buffer |           | buffer  |                  |
michael@0: //                +--------+           +---------+                  |
michael@0: //                | input  |                                        |
michael@0: //     ---------->|        |----------------------------------->    |
michael@0: //       write()  | buffer |  _write()              _receive()      |
michael@0: //                +--------+                                        |
michael@0: //                |                                                 |
michael@0: //                +-------------------------------------------------+
michael@0: 
michael@0: // `_send` is called when more frames should be pushed to the output buffer.
michael@0: Flow.prototype._send = function _send() {
michael@0:   throw new Error('The _send() method has to be overridden by the child class!');
michael@0: };
michael@0: 
michael@0: // `_send` is called by `_read` which is in turn [called by Duplex][1] when it wants to have more
michael@0: // items in the output queue.
michael@0: // [1]: http://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback_1
michael@0: Flow.prototype._read = function _read() {
michael@0:   // * if the flow control queue is empty, then let the user push more frames
michael@0:   if (this._queue.length === 0) {
michael@0:     this._send();
michael@0:   }
michael@0: 
michael@0:   // * if there are items in the flow control queue, then let's put them into the output queue (to
michael@0:   //   the extent it is possible with respect to the window size and output queue feedback)
michael@0:   else if (this._window > 0) {
michael@0:     this._readableState.sync = true; // to avoid reentrant calls
michael@0:     do {
michael@0:       var moreNeeded = this._push(this._queue[0]);
michael@0:       if (moreNeeded !== null) {
michael@0:         this._queue.shift();
michael@0:       }
michael@0:     } while (moreNeeded && (this._queue.length > 0));
michael@0:     this._readableState.sync = false;
michael@0: 
michael@0:     assert((moreNeeded == false) ||                              // * output queue is full
michael@0:            (this._queue.length === 0) ||                         // * flow control queue is empty
michael@0:            (!this._window && (this._queue[0].type === 'DATA'))); // * waiting for window update
michael@0:   }
michael@0: 
michael@0:   // * otherwise, come back when the flow control window is positive
michael@0:   else {
michael@0:     this.once('window_update', this._read);
michael@0:   }
michael@0: };
michael@0: 
michael@0: var MAX_PAYLOAD_SIZE = 4096; // Must not be greater than MAX_HTTP_PAYLOAD_SIZE which is 16383
michael@0: 
michael@0: // `read(limit)` is like the `read` of the Readable class, but it guarantess that the 'flow control
michael@0: // size' (0 for non-DATA frames, length of the payload for DATA frames) of the returned frame will
michael@0: // be under `limit`.
michael@0: Flow.prototype.read = function read(limit) {
michael@0:   if (limit === 0) {
michael@0:     return Duplex.prototype.read.call(this, 0);
michael@0:   } else if (limit === -1) {
michael@0:     limit = 0;
michael@0:   } else if ((limit === undefined) || (limit > MAX_PAYLOAD_SIZE)) {
michael@0:     limit = MAX_PAYLOAD_SIZE;
michael@0:   }
michael@0: 
michael@0:   // * Looking at the first frame in the queue without pulling it out if possible. This will save
michael@0:   //   a costly unshift if the frame proves to be too large to return.
michael@0:   var firstInQueue = this._readableState.buffer[0];
michael@0:   var frame = firstInQueue || Duplex.prototype.read.call(this);
michael@0: 
michael@0:   if ((frame === null) || (frame.type !== 'DATA') || (frame.data.length <= limit)) {
michael@0:     if (firstInQueue) {
michael@0:       Duplex.prototype.read.call(this);
michael@0:     }
michael@0:     return frame;
michael@0:   }
michael@0: 
michael@0:   else if (limit <= 0) {
michael@0:     if (!firstInQueue) {
michael@0:       this.unshift(frame);
michael@0:     }
michael@0:     return null;
michael@0:   }
michael@0: 
michael@0:   else {
michael@0:     this._log.trace({ frame: frame, size: frame.data.length, forwardable: limit },
michael@0:                     'Splitting out forwardable part of a DATA frame.');
michael@0:     var forwardable = {
michael@0:       type: 'DATA',
michael@0:       flags: {},
michael@0:       stream: frame.stream,
michael@0:       data: frame.data.slice(0, limit)
michael@0:     };
michael@0:     frame.data = frame.data.slice(limit);
michael@0: 
michael@0:     if (!firstInQueue) {
michael@0:       this.unshift(frame);
michael@0:     }
michael@0:     return forwardable;
michael@0:   }
michael@0: };
michael@0: 
michael@0: // `_parentPush` pushes the given `frame` into the output queue
michael@0: Flow.prototype._parentPush = function _parentPush(frame) {
michael@0:   this._log.trace({ frame: frame }, 'Pushing frame into the output queue');
michael@0: 
michael@0:   if (frame && (frame.type === 'DATA') && (this._window !== Infinity)) {
michael@0:     this._log.trace({ window: this._window, by: frame.data.length },
michael@0:                     'Decreasing flow control window size.');
michael@0:     this._window -= frame.data.length;
michael@0:     assert(this._window >= 0);
michael@0:   }
michael@0: 
michael@0:   return Duplex.prototype.push.call(this, frame);
michael@0: };
michael@0: 
michael@0: // `_push(frame)` pushes `frame` into the output queue and decreases the flow control window size.
michael@0: // It is capable of splitting DATA frames into smaller parts, if the window size is not enough to
michael@0: // push the whole frame. The return value is similar to `push` except that it returns `null` if it
michael@0: // did not push the whole frame to the output queue (but maybe it did push part of the frame).
michael@0: Flow.prototype._push = function _push(frame) {
michael@0:   var data = frame && (frame.type === 'DATA') && frame.data;
michael@0: 
michael@0:   if (!data || (data.length <= this._window)) {
michael@0:     return this._parentPush(frame);
michael@0:   }
michael@0: 
michael@0:   else if (this._window <= 0) {
michael@0:     return null;
michael@0:   }
michael@0: 
michael@0:   else {
michael@0:     this._log.trace({ frame: frame, size: frame.data.length, forwardable: this._window },
michael@0:                     'Splitting out forwardable part of a DATA frame.');
michael@0:     frame.data = data.slice(this._window);
michael@0:     this._parentPush({
michael@0:       type: 'DATA',
michael@0:       flags: {},
michael@0:       stream: frame.stream,
michael@0:       data: data.slice(0, this._window)
michael@0:     });
michael@0:     return null;
michael@0:   }
michael@0: };
michael@0: 
michael@0: // Push `frame` into the flow control queue, or if it's empty, then directly into the output queue
michael@0: Flow.prototype.push = function push(frame) {
michael@0:   if (frame === null) {
michael@0:     this._log.debug('Enqueueing outgoing End Of Stream');
michael@0:   } else {
michael@0:     this._log.debug({ frame: frame }, 'Enqueueing outgoing frame');
michael@0:   }
michael@0: 
michael@0:   var moreNeeded = null;
michael@0:   if (this._queue.length === 0) {
michael@0:     moreNeeded = this._push(frame);
michael@0:   }
michael@0: 
michael@0:   if (moreNeeded === null) {
michael@0:     this._queue.push(frame);
michael@0:   }
michael@0: 
michael@0:   return moreNeeded;
michael@0: };
michael@0: 
michael@0: // `getLastQueuedFrame` returns the last frame in output buffers. This is primarily used by the
michael@0: // [Stream](stream.html) class to mark the last frame with END_STREAM flag.
michael@0: Flow.prototype.getLastQueuedFrame = function getLastQueuedFrame() {
michael@0:   var readableQueue = this._readableState.buffer;
michael@0:   return this._queue[this._queue.length - 1] || readableQueue[readableQueue.length - 1];
michael@0: };
michael@0: 
michael@0: // Outgoing frames - managing the window size
michael@0: // ------------------------------------------
michael@0: 
michael@0: // Flow control window size is manipulated using the `_increaseWindow` method.
michael@0: //
michael@0: // * Invoking it with `Infinite` means turning off flow control. Flow control cannot be enabled
michael@0: //   again once disabled. Any attempt to re-enable flow control MUST be rejected with a
michael@0: //   FLOW_CONTROL_ERROR error code.
michael@0: // * A sender MUST NOT allow a flow control window to exceed 2^31 - 1 bytes. The action taken
michael@0: //   depends on it being a stream or the connection itself.
michael@0: 
michael@0: var WINDOW_SIZE_LIMIT = Math.pow(2, 31) - 1;
michael@0: 
michael@0: Flow.prototype._increaseWindow = function _increaseWindow(size) {
michael@0:   if ((this._window === Infinity) && (size !== Infinity)) {
michael@0:     this._log.error('Trying to increase flow control window after flow control was turned off.');
michael@0:     this.emit('error', 'FLOW_CONTROL_ERROR');
michael@0:   } else {
michael@0:     this._log.trace({ window: this._window, by: size }, 'Increasing flow control window size.');
michael@0:     this._window += size;
michael@0:     if ((this._window !== Infinity) && (this._window > WINDOW_SIZE_LIMIT)) {
michael@0:       this._log.error('Flow control window grew too large.');
michael@0:       this.emit('error', 'FLOW_CONTROL_ERROR');
michael@0:     } else {
michael@0:       this.emit('window_update');
michael@0:     }
michael@0:   }
michael@0: };
michael@0: 
michael@0: // The `_updateWindow` method gets called every time there's an incoming WINDOW_UPDATE frame. It
michael@0: // modifies the flow control window:
michael@0: //
michael@0: // * Flow control can be disabled for an individual stream by sending a WINDOW_UPDATE with the
michael@0: //   END_FLOW_CONTROL flag set. The payload of a WINDOW_UPDATE frame that has the END_FLOW_CONTROL
michael@0: //   flag set is ignored.
michael@0: // * A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount
michael@0: //   specified in the frame.
michael@0: Flow.prototype._updateWindow = function _updateWindow(frame) {
michael@0:   this._increaseWindow(frame.flags.END_FLOW_CONTROL ? Infinity : frame.window_size);
michael@0: };
michael@0: 
michael@0: // A SETTINGS frame can alter the initial flow control window size for all current streams. When the
michael@0: // value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream by
michael@0: // calling the `setInitialWindow` method. The window size has to be modified by the difference
michael@0: // between the new value and the old value.
michael@0: Flow.prototype.setInitialWindow = function setInitialWindow(initialWindow) {
michael@0:   this._increaseWindow(initialWindow - this._initialWindow);
michael@0:   this._initialWindow = initialWindow;
michael@0: };